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How to manufacture woven polypropylene bulk bags

Synthetic polymer advancements over the years have given rise to new materials with a wide range of applications. Polypropylene (PP) is one such polymer that has proven especially useful in the packaging industry. Bulk bags are made from various elements that help to improve usability, safety and durability. The production process begins with PP pellets which go through an extrusion process to ensure the best quality for consumers.

Step 1: Extrusion

The raw materials (PP, calcium and UV-inhibitors) are combined and melted together before being extruded into long tapes by the extrusion machine. During this process, the PP tapes are regularly checked for quality and durability. These tapes are wrapped around large bobbins before progressing to the next process.

Step 2: Weaving the PP tape

The bobbins of PP tape are loaded onto massive weaving looms, where the bulk bag’s entire structure is woven into formation. This composite material of PP tape is highly resilient and weighs between 65 and 240 grams per square metre. The fabric is also absorbent, enabling moisture transfer through the weave.

Step 3: Optional waterproofing

If a company needs waterproof bulk bags, the woven fabric is exposed to an additional process in which a special laminate coating is applied. This coating prohibits moisture penetration from spoiling water-sensitive granular goods. This process is just one clear example of the adaptability of the bulk bags to the needs of consumers. Since regular, uncoated PP fabric is breathable, it is unsuitable for items such as sugar and salt, which should never be exposed to water, including moisture in the air.

Step 4: Cutting and pressing

Whether or not the woven PP fabric has been treated for moisture, it is placed into the cutting machine and trimmed to precise sizes, based on the customer’s needs. This automated cutting tool uses a stamping technique to cut all of the individual components. The cutting machines are extremely precise. After the cutting and trimming process, the next stage is printing and finishing. 

Step 5: Printing and finishing

The PP fabric is fed into a printing press if a customer wants their logo, contact or safety information displayed on the bag. The printing is often done directly on the cut pieces of PP fabric using specialised printing presses. Most bulk bag manufacturers can print in Pantone colours and match the branding of a company.

Step 6: Straps and handles

The straps used to lift the bulk bags are sewn directly to the fabric. Larger PP tapes are woven together on needle looms to form webbings, which are cut to a particular size and stitched to the bag using high-strength, industrial cotton for added durability. This forms the lifting straps of the bags, which can be fed through a forklift to support the weight of the contents.

Step 7: Sewing and assembly

Tailors bring together and stitch the fabric panels, webbings, spouts and duffle components that collectively form each bulk bag. They use industrial sewing machines and high-strength needles to assemble the pieces of the bulk bags. 

Step 8: Quality control and testing

Trained supervisors thoroughly inspect the bags to ensure there are no flaws. After this, selected bags from each batch are tested for endurance, durability, and reliability. This process ensures that each bag meets the customer’s expectations and safety requirements. Bulk bags that pass quality inspection are compressed and organised into batches for packaging. The bags are wrapped and made ready for shipment. 

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Written by Kate Hawthorne

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